Liquid crystal display devices have been used in various products including watches and electronic calculators, since they are thin and light and consume less electricity. The twisted nematic (TN) display system with the use of nematic liquid crystals is adopted in liquid crystal display devices commonly employed today. Among driving systems for liquid crystal display devices, the simple matrix system, whereby the liquid crystal display device is driven only by electrodes provided on upper and lower substrates, is the most suitable one from the viewpoints of productivity and cost. However, nematic liquid crystals have a slow response time and provide a reduction of the contrast with an increase in the display density, which makes it difficult to construct a high-density display thereby. Accordingly, a very expensive driving system called the active matrix system, wherein each pixel is provided with a thin film transistor (TFT), has been employed in the displays of computers, etc. The production of these displays requires many steps and thus costs a great deal. Thus attempts have been made to cut down the production cost thereof.
On the other hand, Meyer et al. synthesized 2-methylbutyl 4-(n-decyloxybenzylideneamino)cinnamate showing a ferroelectric liquid crystal phase (Sc* phase) in 1975 [R. B. Meyer et al., J. Phys. (France), 36, L69 (1975)]. Further, Clark et al. proposed a surface stabilized ferroelectric liquid crystal device [N. A. Clark et al., Appl. Phys. Lett., 36, 899 (1980)]. Under these circumstances, it is expected that a liquid crystal display device having a fast switching time and an excellent bistability will be produced. Thus a number of ferroelectric liquid crystal materials have been synthesized and proposed so far.
However, the alignment state is much more complicated than expected. Thus, the direction of liquid crystal molecules in a layer is liable to be twisted, which makes it impossible to obtain a high contrast ratio. In addition, it was anticipated that the layer stood perpendicularly to the upper and lower substrates (i.e., a bookshelf structure). In practice, however, the layer is folded (i.e., a chevron structure) and, as a result, zigzag defects arise, which also reduce the contrast ratio. Furthermore, there arises another problem of spontaneous polarization characteristic to ferroelectric liquid crystals. Namely, it has been found that when a memory state is kept for a long period of time, the inversion hardly occurs even under the application of the reverse electrical field (this phenomenon will be hereinafter referred to as ghost effect), thus reducing the contrast.
Recently, there has been reported the existence of a liquid crystal phase by which these problems encountered in ferroelectric liquid crystals might be solved. This liquid crystal phase is an antiferroelectric liquid crystal phase (ScA* phase) having a third stable state in addition to the two stable states (bistability) of the ferroelectric liquid crystal phase. In this third stable state, the molecular tilt direction is reversed between two adjacent layers and thus the spontaneous polarization is drowned out. Although this ScA* phase appears on the lower temperature side of the Sc* phase, the switching time of this ScA* phase is almost comparable to that of the Sc* phase. Moreover, the chevron structure can be switched to the bookshelf structure by changing the applied electrical field. In the ScA* phase, therefore, the bookshelf structure can be easily established by applying an appropriate electric field and thus defects can be eliminated. Further, polarizers and analyzers are provided in such a manner as to make the third state (i.e., the stable state under the application of no voltage) dark and the two ferroelectric states are switched to each other by applying an alternating electric field. Accordingly, no such ghost effect as observed in ferroelectric liquid crystal devices occurs in this case.
In addition, an antiferroelectric liquid crystal phase is also observed in the smectic I phase corresponding to a tilted smectic B phase. Because of being a phase of higher order, it has a low response speed. Thus, the ScA* phase of a low viscosity alone might be applicable to a high contrast display. An antiferroelectric liquid crystal device with the use of this ScA* phase enables simple matrix driving at a low cost and a high productivity. It is said that a display device with a high contrast ratio can be easily achieved thereby.
The antiferroelectric liquid crystal phase was found for the first time in 4-(1-methylheptyloxycarbonyl)phenyl-4'-octyloxybiphenyl-4-carboxylate (hereinafter referred to simply as MHPOBC) represented by the following chemical formula [Chandani et al., Jpn. J. Appl. Phys., 27, L729 (1988)]: ##STR2##
Subsequently, it was clarified that the ScA* phase also appeared when a 1-methylheptyl group in the chiral site was replaced by a 1-trifluoromethylheptyl group. The compound prepared by introducing this 1-trifluoromethylheptyl group exhibits the antiferroelectric liquid crystal phase in a relatively stable state. Accordingly, many derivatives thereof have been reported hitherto as compounds showing the ScA* phase.
In the prior art, a chiral nematic liquid crystal composition or a ferroelectric liquid crystal composition are obtained by adding an optically active compound respectively to a nematic liquid crystal composition or a smectic C liquid crystal composition free from any optically active compound. In the case of an antiferroelectric liquid crystal composition, on the other hand, there have been found few compounds being free from any optically active group and having the same layer structure as that of antiferroelectric liquid crystal phases. Accordingly, an antiferroelectric liquid crystal composition is obtained by constructing a composition of a compound showing an antiferroelectric liquid crystal phase and adding a smectic C liquid crystal compound thereto in such a manner as not to break the layer structure thereof [in general, in an amount of from 30 to 40% (by weight, the same will apply hereinafter)].
However, there have been found few liquid crystal compounds with two rings showing an antiferroelectric phase. That is to say, there has been known no such compound except a cinnamate derivative represented by the following chemical formula, so long as the present inventors know [Proceedings of 18th Liquid Crystal Conference in Niigata, Japan, (1992) 3B419]. ##STR3##
As a compound formally including the liquid crystal compound of the present invention represented by the general formula (I) as will be described hereinafter, JP-A-3-12476 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a phenylpyrimidine derivative represented by the following general formula (II): ##STR4## wherein R.sup.5 and R.sup.6 each represent a straight chain or branched alkyl group having 1 to 18 carbon atoms which can be optionally substituted, provided that at least one of them is an optically active one; X.sup.1 represents --O--, --OC(.dbd.O)--, --C(.dbd.O)O--, or --OC(.dbd.O)O--: and X.sup.2 represents a single bond, --O--, --OC(.dbd.O)--, --C(.dbd.O)O--, or --OC(.dbd.O)O--.
However, the liquid crystal compound disclosed in the above-mentioned patent is not an antiferroelectric liquid crystal compound but a compound which is employed in a ferroelectric chiral smectic liquid crystal composition.
Further, JP-A-4-213387 discloses a phenylpyrimidine derivative represented by the following general formula (III): ##STR5## wherein R.sup.7 and R.sup.8 each represent a straight chain or branched alkyl group having 1 to 18 carbon atoms optionally substituted by an alkoxy group having 1 to 12 carbon atoms, provided that R.sup.7 and R.sup.8 are both optically inactive ones; and Z.sup.1 represents a single bond, --O--, --OC(.dbd.O)--, --C(.dbd.O)O--, or --OC(.dbd.O)O--.
However, this derivative is not an antiferroelectric liquid crystal compound but a compound which shows the smectic C phase and is to be used in a ferroelectric liquid crystal device, similar to the one described above.
When the 1-methylheptyl group in the chiral site of the above-mentioned MHPOBC is replaced by a 1-methylhexyl group, the ferroelectric liquid crystal phase appears alone without showing the ScA* phase. When a 2-methylalkyl group is used, the ferroelectric phase alone appears but no ScA* phase is observed. That is to say, it is highly difficult to structurally modify the chiral site. Although the molecular modification of the chiral site has been discussed, no antiferroelectric liquid crystal phase can be hardly achieved.
There have been generally synthesized various compounds modified in the liquid crystal core side, in particular, the ring structure. Similar to the conventional nematic liquid crystals, antiferroelectric liquid crystals should satisfy various requirements for practical use. These requirements can be hardly satisfied by using a single compound or a group of analogous compounds as described above. Namely, it is required to use a number of compounds differing in properties.
The above-mentioned cinnamate derivative, which is the only compound known as antiferroelectric liquid crystals with two rings, cannot be added to a liquid crystal composition to be used in a liquid crystal display, since it is poor in stability to light. Accordingly, chemically and optically stable two-ring antiferroelectric liquid crystal compounds have never been reported as far as the inventors know. However, in order to obtain a liquid crystal composition having a lower viscosity and a broader temperature range, it is necessary to add a two-ring compound to the composition.
In view of the above, a method of adding a two-ring liquid crystal compound showing a smectic C phase, in which the liquid crystal molecule is tilted in the layer as in the antiferroelectric liquid crystal phase, to the antiferroelectric liquid crystal composition to improve the temperature range and viscosity has been proposed. In this case, however, the antiferroelectric liquid crystal phase would disappear unless a three-ring antiferroelectric liquid crystal compound having higher viscosity such as MHPOBC is added to the composition in an amount of 60 % or more. Accordingly, even in this method, it has been difficult to obtain the desired effects, i.e., broadening the temperature range, lowering the viscosity, etc.
Under these circumstances, it has been urgently required to develop a two-ring compound which is a liquid crystal compound showing chemically and optically stable antiferroelectricity and capable of maintaining the antiferroelectric liquid crystal phase even though added in a large amount to a composition thereof.
Accordingly, the present invention aims at providing a novel optically active compound, which shows an antiferroelectric liquid crystal phase or is highly miscible with known compounds showing an antiferroelectric liquid crystal phase, thus being capable of broadening the temperature range of the antiferroelectric phase, and an antiferroelectric liquid crystal composition containing this optically active compound.
The present inventors have conducted extensive studies on optically active compounds being highly miscible with known compounds showing an antiferroelectric liquid crystal phase and thus capable of broadening the temperature range of the antiferroelectric phase.
In the process of these studies, it has been clarified that the compound disclosed in the above-mentioned JP-A-3-12476, which is one known as showing a ferroelectric chiral smectic C phase or merely a smectic C phase, is unusable as a compound capable of achieving a stable antiferroelectric liquid crystal phase when added to an antiferroelectric liquid crystal compound or an antiferroelectric liquid crystal composition at a ratio of 40% or above. When this compound is added to an antiferroelectric liquid crystal compound or a composition thereof at a ratio of 40% or above, the obtained mixture becomes a smectic A phase liquid crystal composition or a ferroelectric liquid crystal composition as described in the Comparative Examples herein. This is seemingly because the ferroelectric liquid crystal phase would differ from the antiferroelectric liquid crystal phase in layer structure. When ferroelectric liquid crystals or an optically inactive smectic C liquid crystal compound is added at a ratio of 40% or above to a compound showing an antiferroelectric liquid crystal phase, therefore, the layer structure of the antiferroelectric liquid crystal phase would fall into disorder.
Regarding the compound described in JP-A-4-213387, this patent document states nothing about the phase sequence, phase transition temperature, etc., thereof. This compound, which is optically inactive as described in the claims, is one to be used in a ferroelectric liquid crystal device and thus seemingly shows a smectic C phase. Accordingly, it is considered that this compound is also unusable as a compound capable of achieving a stable antiferroelectric liquid crystal phase when added at a ratio of 40% or above to an antiferroelectric liquid crystal compound or an antiferroelectric liquid crystal composition.
Thus, the present inventors synthesized the following compound which is the only one concretely disclosed in the detailed description of the specification of the above-mentioned patent: ##STR6## and examined the phase transition thereof (Comparative Example 7). As a result, it is proved that this compound shows nematic and smectic C phases but not an antiferroelectric liquid crystal phase. It is also found that when this compound is added in an amount of 49.3% by weight to a known compound showing an antiferroelectric liquid crystal phase, the resulting mixture is a liquid crystal composition showing a smectic A phase alone.
On the basis of this discussion, the present inventors have conducted extensive and detailed studies on bicyclic compounds showing an antiferroelectric liquid crystal phase. During these studies, they have successfully found that the compound of the present invention represented by the general formula (I) as will be given hereinbelow, which is an optically active 2-methylalkanoic acid derivative having an alkyl chain of a length falling within a definite range and a carbonate bond, shows an antiferroelectric liquid crystal phase, is highly miscible with known compounds showing an antiferroelectric liquid crystal phase, and exhibits a stable antiferroelectric liquid crystal phase on mixing, thus broadening the temperature range of the antiferroelectric phase in particular toward the lower temperature side. They have furthermore found that the compound represented by the general formula (I) shows either a monotropic antiferroelectric liquid crystal phase or no liquid crystal phase when used alone but provides an antiferroelectric liquid crystal composition when added at a ratio of 60% or above to an antiferroelectric liquid crystal compound or an optically inactive smectic C liquid crystal compound. The present invention has been completed based on these findings. That is to say, the present inventors have found that the compound represented by the general formula (I) has an antiferroelectric liquid crystal phase at around its crystallization temperature (in the process of lowering temperature) or below.